Previous research on rats has established the existence of sex differences in the acquisition and extinction of active and passive avoidance behavior, pain sensitivity, extinction of food rewarded responding, and the acquisition of efficient DRL performance. The present research is designed to examine several possible determinants of sex differences in these behaviors. One series of experiments will examine the effects of manipulating parameters in the behavioral situation to determine the generality of the sex differences across variations in the test conditions. A second series of experiments will experimentswill examine the ontogeny of sex differences in these behaviors. A third series of experimens will study the effects of manipulatin the post-weaning rearing environment on sex differences in thes instrumental behaviors. Fourth, the dependences on gonadal hormones will be examined. Finally, sex differences in h hamsters on the same behavioral tasks will be examined in an attempt to assess the generality of these sex defferences. In all experiments open field activty scores will be correlated with performance in the test situation to assess the relationship between activity and other behavioral differences.